The present invention relates to devices for the synchronization of timing signals and in particular those equipping synchronous data transmission systems.
In general, the succession of binary data is divided up into groups of m bits (m integer.gtoreq.1). A signal element of duration equal to m times the bit period corresponds to each group. This signal element is chosen from two 2m possible forms. The signal elements are transmitted consecutively at a rhythm called the baud rhythm. The passage from one signal element to the next will be called hereinafter "data signal transition".
These systems make it necessary to reconstitute on reception a local timing signal restoring the baud rhythm at which the binary data of a message were transmitted. This local timing signal must be dependent in frequency and phase on the transmission timing signal, so as to give the appropriate value to the data bits on the basis of the data signal received.
A well known solution consists of detecting the transitions of the data signals and dividing the frequency of a timing signal from an oscillator supplying a frequency q times higher than the frequency of the desired local timing signal by means of a divider with a variable dividing number. The value of this variable dividing number is controlled by a control signal supplied by a detector of the position of the data signal transition relative to the rising front of a local timing signal obtained at the output of the divider in order to vary the frequency and consequently the phase of this local timing signal.
This dependence method which uses transitions of the data signal is only possible when data are present. Thus, each new appearance of data makes it necessary to have a prior acquisition period permittting a phase lead of the local timing signal, followed by a synchronization maintenance period on the transitions of the data signals, whilst said signal is present. The acquisition period involves a large number of transitions thus reducing to a minimum the time necessary for the phase lead of the local timing signal.
However, a disadvantage of this solution is caused by the difficulty of having an acquisition period which is both short and reliable. Thus, a short acquisition period when there can only be a limited number of transitions is less reliable than an acquisition sequence using a large number of transitions and permitting a better filtering.
Another disadvantage is due to the slowing down of the synchronization time when the transitions suffer from phase noise and when the starting point of the local timing signal, at the moment when the first transitions appear, is in phase opposition with that which it would have after synchronization.
Another method consists of effecting a prepositioning of the divider and using one of the first data transitions of a message. When the prepositioning has been performed, the phase dependence is maintained on the following data signal transitions.
This solution has the advantage of being very rapid, but can only be used when the quality of the transition channel is sufficient not to transmit interference pulses. However, this does not occur under practical conditions.